.common-bgi() {
  background-repeat: no-repeat;
  background-size: 100% 100%;
}

#box1 {
  width: 100px;
  height: 100px;
  background: url(../image/Desert.jpg);
  .common-bgi();
}
#box2 {
  width: 200px;
  height: 200px;
  background: url(../image/Jellyfish.jpg);
  .common-bgi();
}
#box3 {
  width: 300px;
  height: 300px;
  background: url(../image/gaitubao.png);
  .common-bgi();
}
